The IUPAC name of ${\mathrm{CH}}_3\mathrm{COCH}{\left({\mathrm{CH}}_3\right)}_2$ is

3-Methyl-2-butanone is the name given to this structure by the IUPAC.
The four-carbon chain is the longest. Here, the keto group is the functional group.
Since the functional group should also have the lowest numbering, the numbering will begin with the leftmost carbon.
The third carbon will house the methyl group substituent.
Consequently, this structure's IUPAC designation is 3-Methyl-2-butanone.